The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.
The solenoid valves utilized in such automatic sprinkler systems require energization of the solenoid to operate the valve. Most solenoids used in this application are actuated and held by a 24 volt AC current supplied by the irrigation controller. Typically, one conductor (common) is required to connect each load (solenoid) to each switched current source (output) of the irrigation controller, and one conductor is required to connect the return from all of the loads to the irrigation controller.
In an existing automatic sprinkler system installation, the solenoid operated valves are usually located below ground, and electrical cabling connecting the solenoid operated valves to a reticulation controller will usually be buried underground. When it is desired to add an additional solenoid operated valve, usually there will not be sufficient cabling as the system will have been originally installed with only the required cabling, and without any capacity for expansion.
A previous attempt has been made to operate two independent AC loads from two independent current sources and a return utilizing two conductors to interconnect the current sources and return with the alternating current loads. This product is manufactured by Transitional Systems Manufacturing Company of West Sacramento, Calif., under the trademark “Doubler” and is described in U.S. Pat. No. 4,575,004. The apparatus described in U.S. Pat. No. 4,575,004 is a complex mechanical device incorporating latching switching means, viz. electromechanical latching relays. When this apparatus is used it increases the electrical load on the circuit over and above that formerly presented by the solenoid or solenoids connected in parallel to the irrigation controller. Moreover, such electromechanical relays are subject to malfunction and/or failure over extended periods of time. In addition, the apparatus described in U.S. Pat. No. 4,575,004 cannot be used to switch between two alternating current loads connected thereto unless there is a delay between the first alternating current load being switched off and the second alternating current load being energized. Thus, in most modern irrigation controllers, the apparatus described in U.S. Pat. No. 4,575,004 could not be used to switch between adjacent outputs to solenoid valves in the switching sequence of the irrigation controller.
Another system for operating two independent AC loads from two independent current sources and a return utilizing two conductors to interconnect the current sources and return with the alternating current loads is described in U.S. Pat. No. 5,780,938. A difficulty with this system is that with modern irrigation controllers incorporating monitoring of outputs to solenoid valves, this has resulted in outputs cutting out due to over-current, or outputs being skipped, due to the irrigation controller falsely detecting a fault condition.